1. Field of the Invention
The present invention relates to a scanning optical apparatus and color image-forming apparatus using it and, particularly, is suitably applicable to apparatuses, for example, such as laser beam printers, digital copying machines, multi-function printers, and so on involving the electrophotographic process, which are constructed to deflect a beam emitted from a light source means, by a deflecting element consisting of a rotary polygon mirror and then guide the beam through a scanning lens system having the f-xcex8 characteristic, to optically scan an area on a surface to be scanned, thereby recording image information thereon.
2. Related Background Art
In the conventional scanning optical apparatus used in laser beam printers (LBPs), the digital copying machines, etc., the beam optically modulated according to an image signal and emitted from the light source means is periodically deflected by an optical deflector, for example, consisting of a rotary polygon mirror (polygon mirror), is focused in a spot shape on a surface of a photosensitive recording medium (photosensitive drum) by the scanning lens system having the f-xcex8 characteristic, and is moved to optically scan the area on the surface to record an image thereon.
FIG. 12 is a schematic view of major part of a conventional scanning optical apparatus, for example, proposed in U.S. Pat. No. 6,133,935 (correspondent of Japanese Patent Application Laid-Open No. 10-232347).
In the same figure a divergent beam emitted from light source means 91 is collimated into a nearly parallel beam by collimator lens 92 and the beam (light amount) is limited by stop 93 to enter a cylinder lens (cylindrical lens) 94 having a predetermined refracting power only in the sub scanning direction. In the main scanning section the nearly parallel beam entering the cylinder lens 94 emerges in the nearly parallel beam state as it is. In the sub scanning section the beam is converged to be focused as a nearly linear image on a deflective facet 95a of the optical deflector 95 consisting of the rotary polygon mirror (polygon mirror).
Then the beam deflected and reflected by the deflective facet 95a of the optical deflector 95 is guided through the scanning lens system 106 having the f-xcex8 characteristic, onto the photosensitive drum surface 98 as a surface to be scanned, and the optical deflector 95 is rotated in the direction of arrow A to optically scan the area on the photosensitive drum surface 98 in the direction of arrow B. This implements recording of an image on the photosensitive drum surface 98 being a recording medium.
In the above-stated scanning optical apparatus the scanning lens system 106 is composed of two toric lenses 96, 97 and in the same example all the surfaces (four surfaces) of the two lenses 96, 97 are toric surfaces, thereby correcting various aberrations well.
In general a lens having a large power in the sub scanning direction or an optical element disposed in the vicinity of the optical deflector demonstrates high sensitivity to optical decentration caused during production.
FIG. 13 is a chart showing movement of irradiated position on the surface to be scanned, for example, due to decentration (decentration amount of 0.05 mm) in the sub scanning direction of the optical surfaces of the first toric lens in Example 1 of U.S. Pat. No. 6,133,935. In the specification of the present application the xe2x80x9cdecentrationxe2x80x9d in the sub scanning direction means decentration in the direction normal to the surface to be scanned.
It is apparent from the same figure that the irradiated position moves largely due to the decentration in the sub scanning direction of the toric lens with the largest power in the sub scanning direction and it will pose a problem in recording of high-definition images. This decentration does not affect only the irradiated position but also affects imaging performance. In this optical system a large decentration amount will considerably degrade the spot shape on the surface to be scanned.
This problem may lead to degradation of imagery, because the movement of irradiated position due to decentration causes jitter, pitch unevenness, and color registration, particularly, in the multi-beam scanning optical apparatus for simultaneously scanning the surface with a plurality of beams from a multi-beam light source as a light source, the color image-forming apparatus for guiding beams emitted from a plurality of scanning optical devices, onto image carrier surfaces corresponding to respective colors, to record a color image, and so on.
Further, in the case of optical elements produced by plastic molding or glass molding, in addition to the total decentration of the optical surface, there is a possibility that partial (local) optical decentration can occur depending upon accuracy of mirror finish of a mold. For these reasons, the scanning optical systems with high sensitivity to decentration are not preferable in terms of achievement of higher image quality and improvement in productivity of the scanning optical apparatus. Therefore, there are desires for a scanning optical system permitting good aberration correction and reduced in the sensitivity to decentration.
In the scanning optical apparatus proposed in Japanese Patent Application Laid-Open No. 61-87123, the power in the sub scanning direction is concentrated in an optical element placed in the vicinity of the surface to be scanned, so as to decrease the power in the sub scanning direction of the scanning lens system, thereby reducing the sensitivity to decentration.
The scanning optical apparatus of this reducing system in the sub scanning direction, however, is not suitable for recording of high-definition imagery for the following reasons; a stop normally becomes oblate because of setting of its magnification, coupling efficiency of the collimator lens becomes lower, so as to decrease the light amount, the shape of the stop is close to a rectangle, so as to make the spot shape worse, and so on.
An object of the present invention is to provide a scanning optical apparatus suitably applicable to formation of high-quality imagery with little pitch unevenness, little color registration, or the like by properly setting shapes, powers, etc. of plural optical elements constituting a third optical element for focusing a beam deflected by a deflecting element on a surface to be scanned and thereby reducing the problems of the movement of irradiated position and the degradation of the spot due to the deviation in the sub scanning direction of the third optical element at low cost and in a simple configuration, and also provide a color image-forming apparatus using the scanning optical apparatus.
In one aspect of the invention, there is provided a scanning optical apparatus comprising light source means, a deflecting element for deflecting and reflecting a beam emitted from the light source means, and a scanning optical element for focusing the beam deflected by the deflecting element, on a surface to be scanned,
wherein the scanning optical element comprises a plurality of optical elements,
wherein a shape in a main scanning direction of at least one surface out of optical surfaces of the plurality of optical elements is an aspheric shape,
wherein an optical element having a largest power in a sub scanning direction on the optical axis out of the plurality of optical elements is located on the deflecting element side with respect to a middle point in an optical-axis direction between the deflecting element and the surface to be scanned,
wherein the optical element having the largest power in the sub scanning direction has two optical surfaces,
wherein, where xcfx86S1 represents a power in the sub scanning direction of the optical element having the largest power in the sub scanning direction and xcfx86S1X a power of an optical surface having a smaller power in the sub scanning direction out of the two optical surfaces of the optical element having the largest power in the sub scanning direction, the powers xcfx86S1 and xcfx86S1X satisfy the following condition:
xcfx86S1X less than 0.2xc3x97xcfx86S1.
In a further aspect of the scanning optical apparatus, the optical element having the largest power in the sub scanning direction is located at a position closest to the deflecting element in the optical-axis direction.
In a further aspect of the scanning optical apparatus, only the optical element having the largest power in the sub scanning direction on the optical axis out of the plurality of optical elements is located on the deflecting element side with respect to the middle point in the optical-axis direction between the deflecting element and the surface to be scanned.
In a further aspect of the scanning optical apparatus, a shape in the main scanning direction of at least one surface out of the two optical surfaces of the optical element having the largest power in the sub scanning direction is an aspheric shape.
In a further aspect of the scanning optical apparatus, the power xcfx86S1X of the optical surface having the smaller power in the sub scanning direction out of the two optical surfaces of the optical element having the largest power in the sub scanning direction, is 0.
In a further aspect of the scanning optical apparatus, an optical element other than the optical element having the largest power in the sub scanning direction out of the plurality of optical elements, also has an optical surface a power of which in the sub scanning direction on the optical axis is less than 20% of the power xcfx86S1 in the sub scanning direction of the optical element having the largest power in the sub scanning direction.
In a further aspect of the scanning optical apparatus, concerning all refracting surfaces of the optical elements except for the optical element having the largest power in the sub scanning direction out of the plurality of optical elements, powers thereof in the sub scanning direction on the optical axis are less than 20% of the power xcfx86S1 in the sub scanning direction of the optical element having the largest power in the sub scanning direction.
In a further aspect of the scanning optical apparatus, the optical element having the largest power in the sub scanning direction is a molded lens.
In a further aspect of the scanning optical apparatus, the scanning optical element comprises a diffracting optical element.
In a further aspect of the scanning optical apparatus, shapes in the main scanning direction of both of the two optical surfaces of the optical element having the largest power in the sub scanning direction, are aspheric shapes.
In a further aspect of the scanning optical apparatus, the light source means is a multi-beam light source having a plurality of light emitting portions capable of being optically modulated independently of each other.
In another aspect of the invention, there is provided an image-forming apparatus comprising the scanning optical apparatus as set forth in any one of the foregoing scanning optical apparatuses, a photosensitive member placed at the surface to be scanned, a developing unit for developing an electrostatic latent image formed on the photosensitive member with a beam under scan by the scanning optical apparatus, into a toner image, a transferring unit for transferring the toner image thus developed, onto a transfer medium, and a fixing unit for fixing the toner image thus transferred, on the transfer medium.
In still another aspect of the invention, there is provided an image-forming apparatus comprising the scanning optical apparatus as set forth in any one of the foregoing scanning optical apparatuses, and a printer controller for converting code data supplied from an external device, into an image signal and for supplying the image signal into the scanning optical apparatus.
In still another aspect of the invention, there is provided a color image-forming apparatus comprising a plurality of scanning optical apparatus as set forth in any one of the foregoing scanning optical apparatuses, wherein a plurality of beams emitted from the respective scanning optical apparatus are guided onto a plurality of corresponding image carrier surfaces and areas on the plurality of image carrier surfaces are scanned with the respective beams to form a color image.